The present invention relates to two-wire industrial process control transmitters used to monitor process variables in industrial processes. In particular, the present invention relates to a process variable transmitter which is capable of performing diagnostics on a two-wire process control loop.
Process variable transmitters are used in industrial processes to measure process one or more variables at remote locations in the “field”, and transmit information related to the process variable back to a centralized location such as a control room. Example process variables include pressure, temperature, flow rate, level, etc.
In one configuration, the process variables are transmitted over a two-wire process control loop. The two-wire process control loop connects the process variable transmitter to the control room and, in addition to carrying communication, can also be used to provide power to the process variable transmitter. One example two-wire process control loop is a 4-20 mA process control loop in which the current level ranges between 4 and 20 mA and can be controlled to represent a sensed process variable. Another example process control loop operates in accordance with the HART® (Highway Addressable Remote Transducer) communication protocol. In a process control loop using HART® communication techniques, a digital signal is superimposed on a substantially DC current level carried on the loop. This allows the process control loop to carry both analog and digital signals. The digital signals can be used to transmit additional information from the process variable transmitter to the control room, or transmit data from the control room to the process variable transmitter. Another example two-wire process control loop operates in accordance with a Fieldbus communication protocol in which typically all of the data is carried in a digital format.
If the process control loop is not operating optimally, it is possible for errors to be transmitted by the process variable transmitter, or the loop may provide insufficient power for operation of the process variable transmitter. Other errors including partial failures or total failures may also arise due to problems associated with the two-wire process control loop. Therefore, it is desirable to perform diagnostics on the two-wire process control loop to ensure proper operation. One example of such diagnostics is shown and described in U.S. Pat. No. 5,481,200, entitled FIELD TRANSMITTER BUILT-IN TEST EQUIPMENT issued Jan. 2, 1996 to Voegle et al. and assigned to Rosemount Inc.